The invention relates to the manufacture of cast iron and more particularly to an inoculant for gray cast iron to improve the overall properties thereof.
Cast iron is typically produced in a cupola or induction furnace, and generally has about 2 to 4 percent carbon. The carbon is intimately mixed in with the iron and the form which the carbon takes in the solidified cast iron is very important to the characteristics of the cast iron. If the carbon takes the form of iron carbide, then the cast iron is referred to as white cast iron and has the physical characteristics of being hard and brittle which in certain applications is undesirable. If the carbon takes the form of graphite, the cast iron is soft and machine-able and is referred to as gray cast iron.
Graphite may occur in cast iron in the flake, vermicular, nodular or spherical forms and variations thereof. The nodular or spherical form produces the highest strength and most ductile form of cast iron.
The form that the graphite takes as well as the amount of graphite versus iron carbide, can be controlled with certain additives that promote the formation of graphite during the solidification of cast iron. These additives are referred to as inoculants and their addition to the cast iron as inoculation. In cast iron production the foundries are constantly plagued by the formation of iron carbides in thin sections of the castings. The formation of iron carbide is brought about by the rapid cooling of the thin sections as compared to the slower cooling of the thicker sections of the casting. The formation of iron carbide in a cast iron product is referred to in the trade as “chill”, The formation of chill is quantified by measuring “chill depth” and the power of an inoculant to prevent chill and reduce chill depth is a convenient way in which to measure and compare the power of inoculants.
As the industry develops there is a need for stronger materials. This means more alloying with carbide promoting elements such as Cr, Mn, V, Mo etc., and thinner casting sections and lighter design of castings. There is therefore a constant need to develop inoculants that reduce chill depth and improve machinability of gray cast iron.
Since the exact chemistry and mechanism of inoculation and why inoculants function as they do, is not completely understood, a great deal of research goes into providing the industry with a new inoculant.
It is thought that calcium and certain other elements suppress the formation of iron carbide and promote the formation of graphite. A majority of inoculants contain calcium. The addition of these iron carbide suppressants is usually facilitated by the addition of a ferrosilicon alloy and probably the most widely used ferrosilicon alloys are the high silicon alloy containing 75 to 80% by weight silicon and the low silicon alloy containing 45 to 50% by weight silicon.
U.S. Pat. No. 3,527,597 discovered that good inoculating power is obtained with the addition of between about 0.1 to 10% by weight strontium to a silicon-bearing inoculant which contains less than about 0.35% by weight calcium and up to 5% by weight aluminum. U.S. Pat. No. 3,527,597 is incorporated herein by reference.
U.S. Pat. No. 4,749,549 provided an inoculant consisting essentially of about 15 to 90% by weight silicon, about 0.1 to 10% by weight strontium, less than about 0.35% by weight calcium, up to about 5% by weight aluminum, not more than about 30% by weight copper, one or more additives selected from about 0.1 to 15% by weight zirconium and about 0.1 to 20% by weight titanium, and a balance of iron, with residual impurities in the ordinary amount. U.S. Pat. No. 4,749,549 is incorporated herein by reference.
Also a method for making an inoculant for cast-irons by adding a strontium rich material and material rich in one or more additives selected from zirconium, titanium alone or in combination to a molten ferrosilicon low in calcium at a sufficient temperature and for a sufficient period of time to cause the desired amount of strontium to enter the ferrosilicon is provided in U.S. Pat. No. 4,666,516, which is incorporated herein by reference.
Superseed® Extra inoculant, a ferrosilicon alloy with (1.0-1.5% by weight Zr, 0.6-1.0% by weight Sr, 0.1% max by weight Ca and less than 0.5% by weight Al) has been used successfully for several years to make thin walled, high strength gray iron castings.
However, for some cast irons it is desirable to increase the aluminum content of the cast iron to at least 0.01% by weight in order to reduce chill in thin walled gray iron castings. In order to achieve this, Alinoc® inoculant (a ferrosilicon alloy with 3.5-4.5% by weight Al, 0.5-1.5% by weight Ca) has been added to the cast iron in the transfer ladle to increase aluminum content of the cast iron followed by addition of Superseed® Extra inoculant in the pouring ladle to reduce chill in new generation, thin walled gray iron castings.
However, this has shown to create problems due to slag build up in the pouring unit probably caused by the high calcium content in the Alinoc® inoculant. The pouring unit can thus only be used for a limited number of cast iron melts and thus adds to the costs for producing cast iron products. There is thus a need for an inoculant with a higher aluminum content and a low calcium content that can be used as the only inoculant added to the cast iron in the transfer ladle, in the pouring unit or in the molten cast iron stream.